The manufacture of dental prostheses such as crowns and bridges needs to be as precise as possible in order to ensure that, externally, the crown fits within the area of the oral cavity assigned thereto. It is also important to ensure, of course, that internally, and also with respect to the margin line, the crown fits properly onto the preparation.
Referring to the external profile of the crown, the width of the crown must be such so as not to interfere with the adjacent teeth, and to thus maintain a reasonable interproximal gap on either side of the crown with neighboring teeth. Moreover a proper functional contact relationship needs to be established with respect to the surrounding teeth, both for static and dynamic conditions. Thus the heights and shapes of different parts of the crown must be controlled accordingly. Further, close fitting contact is also required between the lower edge of the crown and the finish line of the preparation to ensure long and trouble-free life for the prosthesis, which in turn requires the lower edge to follow closely the profile of the finish line. Manual manufacturing methods for dental prostheses typically require a number of “fitting and fixing” cycles in order to ensure that the crown is properly dimensioned before finally fixing the prosthesis to the preparation in the intraoral cavity.
CNC-based methods for manufacturing dental prostheses are known and represent a significant improvement in automating the manufacturing process to provide a high degree of dimensional accuracy. For example, in U.S. Pat. No. 4,663,720 and in U.S. Pat. No. 5,027,281, material is removed from a massive block of material by means of a CNC milling machine, and the machining paths are calculated from a 3D numerical model of prosthesis. In U.S. Pat. No. 4,937,928, a dental prosthesis is manufactured by successively applying a number of layers of material on a model in the shape of the part of the teeth where the prosthesis is to be provided. After each layer is applied, the workpiece is worked by a CNC tool controlled by a CAD/CAM system. In U.S. Pat. No. 5,378,154, a similar method is used for forming layers of material onto a preparation, machining each layer along paths that follow three dimensional irregularly spaced curved lines.
In each case, the machining paths are predefined without reference to the pre-machining geometry of the material layers.